1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Description of the Related Art
With the recent tendency toward advancing TV use of liquid crystal display devices, the panel size of the devices is enlarged and high-definition and low-price liquid crystal display devices are much desired. In particular, VA-mode liquid crystal display devices have a relatively high contrast and enjoy a relatively high production yield, and have therefore become most popular liquid crystal display devices for TV use.
However, liquid crystal display devices, especially VA-mode liquid crystal display devices have a problem in that, at the time of black level of display, the devices could provide black display that is almost complete in the normal direction to the display panel, but when the black level panel is seen in oblique directions, there occurs light leakage to disable background black display whereby the viewing angle is narrowed.
For solving the problem of viewing angle characteristics, for example, there has been proposed a method of incorporating a combination of a first retardation plate having a positive monoaxial refractivity anisotropy and a second retardation plate of which the in-plane refractive index is sufficiently smaller than the thickness-direction refractive index thereof and which has a negative refractivity anisotropy, into a liquid crystal display device to thereby reduce the light leakage in watching the panel of the liquid crystal display device in oblique directions at the time of black level of display (for example, Patent Reference 1).
However, the method described in Patent Reference 1 is for reducing the light leakage only within a specific wavelength region (for example, green light at around 550 nm), and therefore in the method, there still occurs light leakage in the other wavelength region in which the wavelength dispersion characteristics of the retardation plates and the wavelength dispersion characteristics of the liquid crystal cell in the device are incompatible with each other (for example, blue light at around 450 nm and red light at around 650 nm). Accordingly, when the black display panel of the liquid crystal display device described in Patent Reference 1 is watched in oblique directions, then the panel is colored in blue or red, or that is, there occurs a problem of color shift. Consequently, the method described in Patent Reference 1 is still unsatisfactory in point of solving the problems of viewing angle characteristics.
As a method for solving the problem of color shift in watching the black display panel of the liquid crystal display device in oblique directions, improving the wavelength dispersion characteristics of the film for use in the retardation plate in the device has become investigated in order that the wavelength dispersion characteristics of the retardation plate and the wavelength dispersion characteristics of the liquid crystal cell in the device could be compatible with each other. As a method of improving the wavelength dispersion characteristics of a retardation plate, there has been disclosed a method of stretching a film of a cellulose acetate of which the degree of acylation falls within a specific range to thereby provide a cellulose acetate film having a larger birefringence at a longer wavelength (see Patent Reference 2).
On the other hand, Patent Reference 3 and Patent Reference 4 disclose a cellulose ester laminate film having a specific laminate structure and having a lowered haze. In these patent references, various characteristics of the film, such as the moisture permeability and the dimensional stability in addition to the haze thereof are investigated, however, nothing is investigated therein relating to the wavelength dispersion characteristics of retardation of the film.
As another method of solving the problem of color shift, there has been investigated a multigap configuration of a liquid crystal cell. This is a method of changing the thickness of the color filter in every pixel region of R, G and B (for example, the thickness of the color filter in the pixel region of R, G and B is settled as R pixel region<G pixel region<B pixel region) to thereby change the thickness (gap) of the liquid crystal layer in every pixel region of R, G and B (in this case, the thickness of the liquid crystal layer in each pixel region of R, G and B is: R pixel region>G pixel region>B pixel region). According to the method, the problem of color shift could be solved in some degree. For example, Patent Reference 5 describes a case where the thickness of the color filter in every color pixel of R, G and B is defined as R pixel region>G pixel region>B pixel region and the color filter is combined with a film having predetermined optical properties, thereby improving the viewing angle characteristics and the panel contrast.